INTRODUCTION

Lemon sharks are nocturnal predators that feed on a variety of fish and invertebrates (Newman et al. 2004).  Due to this, once a prey stimulus is encountered, N. brevirostris relies heavily on olfaction and electroreception.  Several centimeters prior to feeding, N. brevirostris exposes its nictitating membrane, to protect its eye from damage while feeding (Frazzetta & Prange, 1987).  During this time, it is thought that only its electric sense is being used to detect its prey item.  The ability of this shark to detect the electric impulses from its prey items occurs through the sharks electroreceptors known as the Ampullae of Lorenzini.  The sensory cells within the electroreceptors detect a potential difference between the ampullae and the prey item.  Once this potential difference is recognized an impulse is sent to the brain via the afferent neurons.  This will allow the shark to home in on the prey item and feed (Kalmijn, 1971; Murray, 1962).

 We placed eight (8) N. brevirostris in a pen that was divided by our 3-dimensional net-like apparatus.  At one side of the apparatus, 30 cm away from the edge of the pen, a 70 cm x 70 cm hole was cut with no stimulus surround the hole (control).  On the other side of the apparatus, 30 cm away from the edge of the pen, another 70 cm x 70 cm hole was cut with magnets surround the hole (treatment).  With this experimental setup, one can assume that these sharks will enter the control hole a greater amount of times than the treatment hole.  Also, one may also predict that the sharks will display a greater amount of avoidance behaviors in response to the treatment hole.

MATERIALS AND METHODS

For this experiment, we used 8 juvenile (72-88 cm) N. brevirostris that were used in a prior experiment.  These sharks were placed in a 6 m diameter holding pen.  The same experimental setup used in the juvenile tiger shark (Galeocerdo cuvier) magnetic barrier experiment was employed.  Each N. brevirostris contained a passive integrated transponder (PIT) tag and was identified by using a PIT tag reader.  Each shark was chosen at random from their holding pen and was first tested with the control on the northern region of the pen, while the treatment was located near the southern region of the pen.  Once these trials were completed, each shark was randomly chosen and tested with the location of the control and treatment switched (control = southern region, treatment = northern region).  The control and treatment were switched after each shark completed the first experimental setup in order to see if there was a side bias during our experiment. 

To begin the trial, we placed one N. brevirostris into the pen and began to record data for a duration of one hour as soon as the shark stopped accelerating.  We noted the number of times the shark entered the square holes along with a variety of other behaviors.  An entrance was recorded when the shark passed completely through the square opening.  Other behaviors that were noted were:  approaches, 90¥ turns, 180¥ turns, accelerations, and bumps.  Approaches were recorded when the shark swam directly at the square or when the shark swam along the contour of the pen and then began to swim into the hole (i.e. sharks head broke the plane of the square).  There were two types of turns that were recorded, 90¥ turns and 180¥ turns.  A 90¥ turn was recorded when the shark swam towards the square and made a right-angled turn at a distance equal to or less than a half meter from the square.  A 180¥ turn was recorded when the shark swam towards the square and made a complete u-turn at a distance equal to or less than a half meter from the square.  Accelerations were recorded when the shark was within a half meter of the control or treatment square and quickly increased its speed in a direction away from either of the squares.  Bumps were recorded when the shark made contact with the edges of the square using its head.  

PRELIMINARY RESULTS (OCTOBER 2006)

Two, one hour trials were conducted on each N. brevirostris and all behaviors of each shark were noted.

Our results display that approaches were significantly more frequent towards the control square as compared to the treatment square.  Avoidance behaviors (accelerations away from, complete avoidance, 90¥ turns and 180¥ turns) and entrances for both the control and treatment squares, showed variation but not enough to be significant.  (Tables 1 and 2; Figures:  1 and 4).  Lastly, there was a slight variation in the side preference in which the shark entered, but no significance was found (Figure 2).

DISCUSSION OF RESULTS (OCTOBER 2006)

Although the majority of our results were not significant, with slight modifications, our results may show that permanent magnets have a large impact on N. brevirostris.  This preliminary experiment showed that these magnets are having an impact on N. brevirostris, but a modification that can be made would be a second magnetic barrier approximately 10 cm away from the first.  Similar to G. cuvier, when N. brevirostris entered the treatment region, the shark decelerated. This behavior was observed in seven out of the nineteen entrances into the treatment zone.  This observation could suggest that the electric fields produced by the magnets are in fact being detected by N. brevirostris.  With only one field being present, the lemon shark can potentially push through this field.  But, by placing a second magnetic barrier, thus another electric field, it is possible that as the shark decelerates once he passes the first magnetic barrier, the shark will then turn around as soon as he detects the second magnetic barrier.  It is pertinent that these trials continue with the inclusion of the second magnetic barrier so we can increase our chances of completely deterring these sharks. 

Also, it was found that as the trials progressed, the amount of avoidance behaviors and entrances increased.  This may also be an indication that the sharks are slightly stressed in the beginning of the trials, but acclimate to their surroundings by the end of the trials.  This suggests that the trials should be longer in duration to enable us to get more accurate results.

Negaprion brevirostris was found to significantly approach the control side of the pen more frequently than the treatment side.  This is important because this could potentially indicate that N. brevirostris is extremely sensitive to these magnets and does not like to be in the magnetic region of the pen.  Due to this, it may be important to put a hose divider perpendicular to the fence.  This will allow us to have 4 quadrants (2 control and 2 stimulus quadrants).  This will enable us to record the amount of time the shark spends in a particular side of the pen (control versus treatment) and will give us the ability to quantify whether or not the shark spends a greater amount of time on the control side of the pen compared to the treatment side. 

In conclusion, our data does not firmly suggest that magnets affect N. brevirostris behavior.  We cannot assume that the magnets can deter N. brevirostris because there was not a sufficient amount of trials.  With more experiments, we may discover that sharks avoid these permanent magnets, which could have implications on the behavior of other sharks to permanent magnets.